Vacuum-UV fluorescence spectroscopy of PF3 in the range 9–20 eV

The vacuum-UV and visible spectroscopy of PF3 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following photoexcitation with monochromatized synchrotron radiation from the Daresbury, UK source in the energy range 9–20 eV with an average resolution of ;0.015 eV. Transitions to the three lowest-energy bands in the Rydberg spectra show resolved vibrational structure, they are assigned to transitions to the (8a1)21 4p, 5p, and 6p Rydberg states of PF3, and fluorescence is due to valence transitions in the PF2 radical. From a Franck–Condon analysis of the vibrational structure, it is shown that the FPF bond angle in PF3 increases by ;1461° upon photoexcitation. The use of optical filters shows that at least three excited electronic states of PF2 are responsible for the induced emission. Dispersed emission spectra in the UV/visible region have been recorded with an optical resolution of 8 nm at the BESSY 1, Germany synchrotron source at the energies of all the peaks in the excitation spectrum. Four different decay channels are observed: ~a! PF2 A ˜ 2A1 –X ˜ 2B1 fluorescence in the wide range 320–550 nm for photon energies around 9.8 eV, ~b! PF2 A ˜–X ˜, and B ˜ 2B2 –X ˜ 2B1 fluorescence at ;300 nm for photon energies around 11.0 eV, ~c! PF2 C˜ 2A1 –X ˜ 2B1 and E ˜ 2B1 (2P) –A ˜ 2A1 fluorescence at ;222 and 325 nm, respectively, for photon energies around 14.4 eV, and ~d! PF A 3P–X 3S2 fluorescence between 300–380 nm for photon energies around 16.1 eV. These assignments are confirmed by action spectra in which the excitation energy of the vacuum-UV radiation is scanned with detection of the fluorescence at a fixed, dispersive wavelength. Using the single-bunch mode of the BESSY 1 source, we have attempted to measure the lifetimes of the emitting states, but the timing profile of the source imposes an upper limit on lifetimes that can be measured of ;500 ns. We have therefore only been able to measure values for the bent C˜ 2A1 and linear E ˜ 2B1 (2P) states of PF2 of 14.7 and 7.9 ns, respectively; the lifetimes of the other emitters are too long to measure by this method. Our assignments in PF2 are heavily dependent on recent ab initio calculations on the geometries and energies of the valence electronic states of this species. Our knowledge on the electronic spectroscopy of this free radical is reviewed.